1. Field of the Invention
This invention relates to a modification of a self-excited type DC/DC converter, particularly to a modification of a ringing choke converter.
2. Description of the Related Art
Ringing choke converters are inexpensive devices, generally requiring a small number of components. Since they demonstrate stabilized DC characteristics, they are often used as power supplies for general appliances using electric circuits, such as VTRs.
FIG. 1 shows an example of an existing ringing choke converter. In this example, one end of a primary winding L1 of a transformer T is connected to one input terminal A. The other end of the primary winding L1 is connected to the collector of a switching transistor Q1. The emitter of the switching transistor Q1 is connected to the other input terminal B. The output of a secondary winding L2 of the transformer T is connected to the output terminals X, Y via a rectifying/smoothing circuit including a diode D1 and a capacitor C1.
One end of a base winding L3 of the transformer T is connected to the base of the switching transistor Q1 via a base-driving device consisted of a series circuit including a resistance R2 and a capacitor C2. The other end of the base winding L3 is connected to the emitter of the switching transistor Q1. The base of the switching transistor Q1 is connected to the input terminal A via a starting resistance R1. The switching transistor is turned on and off repetitively by the self-excited oscillation of this base-driving system.
The description given above explains a well-known structure of a ringing choke converter. However, this basic circuit system is not designed to provide a constant output voltage V.sub.out. Therefore, a system adapted to control the timing to turn off the switching transistor Q1 is provided in order to stabilize the output voltage V.sub.out. This control system variably controls the turn-off timing in accordance with the output voltage V.sub.out detected either directly or indirectly. In FIG. 1, the timing-control system comprises a controlling transistor Q2 connected between the base and the emitter of the switching transistor Q1, and a feedback controlling circuit 10 for driving the controlling transistor Q2. When the controlling transistor Q2 is turned on upon receipt of an output signal from the feedback controlling circuit 10 while the switching transistor Q1 is turned on, the base current of the switching transistor Q1 is taken by the controlling transistor Q2, and thereby the switching transistor Q1 is forcedly turned off. Therefore, there is provided a feedback control mechanism which is capable of adjusting the ON duration of the switching transistor Q1, and capable of maintaining the output voltage V.sub.out at a constant level.
It is well known in the art that if the input and output voltages are kept at a constant level, the oscillating frequency of a ringing choke converter is inversely proportional to the load current, i.e., the frequency is increased as the load becomes smaller. However, a ringing choke converter is generally designed so that a predetermined amount of electric power is supplied when the load is at its maximum value. If the minimum load were to be 1/100 of the maximum load, the switching frequency at minimum load current would, theoretically, amount to 100 times the frequency at maximum load. High frequency usually results in increase in switching losses, decrease in efficiency, and causes undesirable radiation noise problems.
In a ringing choke converter employed as a power supply for VTRs or other such devices, there is quite a difference between the minimum load during the standby-mode and the maximum load during the normal playing-mode. Therefore, the switching frequency during the standby-mode becomes exceedingly high, thereby causing problems such as reduction in efficiency and increase of noise.